Vladimiro Montesinos wove a huge web of corruption
The Attorney-General, Nelly Calderon, said she had opened a preliminary investigation into Antonio
Ketin Vidal following a report from the US Treasury on suspicious transactions in American banks between accounts held under his and Mr Montesinos' names.
"This investigation does not imply an accusation, but an investigation to discover the truth," Mrs Calderon told a news conference.
US Treasury intelligence sources have traced what they call "suspicious operations" in bank accounts involving the two men in 1995 and 1996.
Mr Vidal denies any wrongdoing, saying he has never had an account outside Peru.
Surprise
"This is surprising news for me," Mr Vidal told reporters. "I have nothing to do with any account, or with Vladimiro Montesinos."
Mr Montesinos, who is currently in jail awaiting trial in Lima, was captured in Venezuela and returned to Peru after eight months on the run.
He fled the country when videos came to light which allegedly showed him bribing politicians, judges and other top officials when Alberto Fujimori was in power between 1990 and 2000.
The scandal led to President Fujimori's downfall in November last year, and Mr Montesinos' flight to Venezuela.
It was he who personally brought back Mr Montesinos from Venezuela in June of this year.
National hero
He had previously become a national hero in Peru when he masterminded the capture of Abimael Guzman, the leader of the Sendero Luminoso guerrilla group, in 1992.
Mr Ketin Vidal stepped down from government when the new president Alejandro Toledo took office on 28 July.
More than 500 people are being investigated for alleged illegal links to Mr Montesinos.
About 100 people, including several members of Mr Montesinos' family, have been arrested on corruption charges.
Reports from Peru say that Mr Montesinos himself may be brought to trial in the next few weeks, on the first of more than 50 charges, ranging from murder to money laundering.